iOS Swift MapKit making an annotation draggable by the user?

Question

How do I make it possible, using MapKit in Swift, for the user to drag an annotation from one position to another within the map? I have set the annotation view to be draggable, when my map view delegate creates the annotation view, like this:

func mapView(_ mapView: MKMapView, viewFor annotation: MKAnnotation) -> MKAnnotationView? {
    var v : MKAnnotationView! = nil
    if annotation is MyAnnotation {
        let ident = "bike"
        v = mapView.dequeueReusableAnnotationView(withIdentifier:ident)
        if v == nil {
            v = MyAnnotationView(annotation:annotation, reuseIdentifier:ident)
        }
        v.annotation = annotation
        v.isDraggable = true
    }
    return v
}

The result is that the user can sort of drag the annotation - but only once. After that, the annotation becomes impossible to drag, and even worse, the annotation now no longer "belongs" to map - when the map is scrolled / panned, the annotation holds still rather than scrolling / panning with the map. What am I doing wrong?

Answer 1

It isn't enough to mark the annotation view by setting isDraggable to true. You must also implement mapView(_:annotationView:didChange:fromOldState:) in your map view delegate - and (even more important) this implementation must not be empty! Rather, your implementation must, at a minimum, communicate the drag state from the incoming parameters to the annotation view, like this:

func mapView(_ mapView: MKMapView, annotationView view: MKAnnotationView, didChange newState: MKAnnotationViewDragState, fromOldState oldState: MKAnnotationViewDragState) {
    switch newState {
    case .starting:
        view.dragState = .dragging
    case .ending, .canceling:
        view.dragState = .none
    default: break
    }
}

Once you do that, the annotation will be properly draggable by the user.

(Many thanks to this answer for explaining this so clearly. I can't claim any credit! My answer here is merely a translation of that code into Swift.)